1. Technological Field
The present disclosure relates to video calls, in particular to an Integrated Services Digital Network (ISDN) device capable of handling multiple video calls.
2. Description of the Related Art
The H.320 standard from the International Telecommunications Union (ITU) is a common standard for multimedia communications over circuit switched networks, such as the Integrated Services Digital Network (ISDN). H.320 is an umbrella standard with several sub standards defining the different protocol layers. For example, H.221 is the framing protocol of H.320. Since video calls require more bandwidth than a single ISDN channel, several channels are set up to make a combined higher bandwidth communication link. The channels making up such a communication link are conventionally called Master Call and Sub-Calls. The Master Call is the first channel to be established, while the Sub-Calls are the subsequent channels to be established. The Master Call and the Sub-Calls are all dialed independently of each other, from the “calling” endpoint to the “called” endpoint.
The H.221 Aggregation protocol defines directly sending H.221 frames in each channel. These frames provide a Sub-Call identifier (channel number, n) for each channel participating in a point-to-point video call. However, if a receiving endpoint receives video calls from several transmitting endpoints, it is not possible to identify which Sub-Calls belong to which connection. As the receiving endpoint cannot reliably recreate the communication links, the receiving end will drop Sub-Calls or entire connections altogether.
One possible solution may be for the receiving endpoint to only accept one incoming connection at a time. In this case, the endpoint needs to immediately drop channels with a Sub-Call Identifier n=1 (indicating a new Master Call) until all the Sub-Calls have been established for the current incoming connection. This would provide a poor user experience, forcing callers to dial again until their connection gets accepted.
Another possible solution may be using the Calling Number information sometimes available from the call control layer (ITU Recommendation Q.931 protocol), and considering all ISDN channels with identical Calling Numbers being part of the same connection. However, the provision of this information relies on optional supplementary services (i.e., Calling Line Identification Presentation (CLIP)) which may not be available, this information may be hidden (Calling Line
Identification Restriction (CLIR)), and networks may modify or strip this information. Furthermore, different connections may have identical Calling Numbers (e.g., multiple connections from a Gateway to another Gateway), or Calling Numbers within one connection may be different (e.g., ISDN channels spanning different Primary Rate Interfaces (PRIs)).
Another possible solution is the BONDING™ protocol, of the BONDING™ consortium, which solves the problem by sending a Connection Identifier (the Group ID) as well as a Sub-Call Identifier (the channel ID), in-band, within each ISDN channel. The Connection Identifiers are allocated by the receiving endpoint, and kept unique across all incoming connections. Each calling endpoint transmits its own Connection Identifier within each of its Sub-Calls, to allow the receiving endpoint to identify which ISDN channels are used by remote endpoints. The Sub-Call Identifiers, all different within a given connection, are used by the receiving endpoint to reorder the ISDN channels. This technique, therefore, provides an ISDN endpoint (capable of handling multiple connections), or an ISDN Gateway, the ability to receive simultaneous incoming BONDING™ connections. FIG. 1 illustrates two remote endpoints, Endpoint A 100 and Endpoint B 110, which are calling a Gateway 130 across an ISDN network 120. In FIG. 1, the first digit (of the x/x identifiers) represents the Connection Identifier, and the second digit represents the Sub-Call Identifier sent within each ISDN channel.
However the BONDING™ protocol may not be supported by older endpoints. The BONDING™ protocol requires all channels that are part of a connection to be synchronized before media can be conveyed. However, the H.221 Aggregation protocol allows media to be conveyed while new ISDN channels are still being added to a connection.